Background Treating an anterior cruciate ligament (ACL) injury in pediatric patients is challenging. The GNRB® is a diagnostic tool for laximetry and can provide valuable information for clinical decision. As few data exist on this subject, the first objective is to provide physiological laxity values of anterior tibial translation in healthy and pathological knees in pediatric population by age and gender groups. A second objective is to establish injury thresholds with the GNRB®. Methods This prospective study included 141 pediatric patients with a complete ACL tear. They all underwent GNRB® measurements before the surgery. Age and gender groups were created: child/adolescent girls and child/adolescent boys. Thresholds were established using ROC curves and 2-factors ANOVA tests were used for comparisons. Results A significant age effect was found for laxities measured on healthy knees. Significant age and gender effects were found for laxities measured on pathological knees and for side-to-side differences of laxity (SSD) between healthy and pathological knees. The optimal threshold to detect a complete ACL rupture with GNRB® is 1.5 mm at 134N with a sensitivity of 83.7% and a specificity of 92.7%. Conclusion GNRB® brought new information about ACL laxities for healthy and pathological knees in pediatric population. Children have a greater anterior tibial translation which decreases gradually with age, and girls have grater laxities than boys. Thresholds for complete ACL tears, presenting good predictive values, were established and can help surgeons in their decisions to improve clinical practice. Level of evidence: Prospective cohort study, level of evidence III.